Field of the Invention
The present invention relates to an image forming apparatus, which is configured to correct distortion and uneven image density of an image during image formation of a two-dimensional image by the image forming apparatus, e.g., a digital copying machine, a multifunctional peripheral, or a laser printer.
Description of the Related Art
In electrophotographic image forming apparatus such as a laser printer and a copying machine, there has been generally known a configuration to form a latent image on a photosensitive member with the use of a light scanning device configured to perform scanning with a laser beam. In the light scanning device of a laser scanning type, a laser beam collimated with the use of a collimator lens is deflected by a rotary polygon mirror, and the deflected laser beam is formed into an image on a photosensitive member with the use of an elongated fθ lens. Further, there is known multibeam scanning in which a laser light source having a plurality of light emitting points is included in one package so as to perform scanning with a plurality of laser beams simultaneously.
Meanwhile, in order to form a satisfactory image without uneven image density and banding, it is desired that distances between scanning lines of which positions to be scanned with a laser beam are adjacent to each other in a rotational direction of the photosensitive member be equal to each other. However, the distances between the scanning lines are varied due to a plurality of factors described below. The distances between the scanning lines on the photosensitive member are varied by, for example, a fluctuation in a surface speed of the photosensitive member, or a rotation speed fluctuation of a rotary polygon mirror. Further, the distances between the scanning lines are also varied by a variation in angle of mirror faces of the rotary polygon mirror with respect to a rotary shaft of the rotary polygon mirror and a variation in intervals between light emitting points arranged on a laser light source. To cope with uneven image density and banding caused by such factors, there has been proposed a technology of correcting banding by controlling an exposure amount of the light scanning device. For example, in Japanese Patent Application Laid-Open No. 2012-098622, there is described a configuration in which a beam position detection unit configured to detect a beam position in a sub-scanning direction is arranged in the vicinity of the photosensitive member, and the exposure amount of the light scanning device is adjusted based on scanning distance information obtained from a detected beam position, to thereby make banding less noticeable.
Due to factors such as a variation in dimensions of components forming the image forming apparatus, a laser beam emitted from a light scanning device has positional deviation from the position of an ideal focus in a focus depth direction. The amount of banding changes when the positional deviation occurs in the focus depth direction. FIG. 17A is a diagram for illustrating a state of an optical path formed based on the inclination of a mirror face of a rotary polygon mirror. For example, as illustrated in FIG. 17A, when a photosensitive member is deviated by ±α from a distance Lf which is a predetermined distance, a positional deviation of ±Δd occurs in a sub-scanning direction. As shown in FIG. 17C, the amount of optical face tangle of a just focus correction position is corrected by a lens 1001, but the amount is as shown in FIG. 17B or FIG. 17D at the position deviated by ±α from the distance Lf. Thus, when the positional deviation occurs in the focus depth direction, positional deviation is caused in the sub-scanning direction to change the amount of banding.
In the related art, when the positional deviation occurs in the focus depth direction due to the factors such as a variation in dimensions of components, a correction residual occurs because of an error in a correction amount. Even a configuration including a position detection unit configured to detect the position of a light beam has a problem in that, when the position of the position detection unit in the focus depth direction differs from the position of the photosensitive member in the focus depth direction due to, for example, a mounting error of the position detection unit, a detection error occurs, and banding cannot be eliminated completely.